The present invention is directed to an improved composite susceptor material for use in the manufacture of disposable packages for cooking in microwave ovens. More specifically, the present invention comprises an improved composite structure including paperboard, which when incorporated into packages for heating food in a microwave oven, provides cooked food products with a desirable crisp outer surface and a cooked but moist interior. By incorporating at least two spaced susceptor layers in the composite structure in overlying relation, the susceptor layers provide both a shielding and a heating effect. Thus while the prior art discloses susceptor materials for use in the manufacture of packages for heating the surfaces of food products in a microwave oven, and package structures for shielding specific surfaces of food products while heating other surfaces, the present invention provides a single material with both advantages, namely, intense heating at the surface combined with shielding of the interior of the food products.
U.S. Pat. Nos. 4,641,005 and 4,825,025 each teach the construction of a single susceptor material for use in making disposable packages for use in a microwave oven. In each case, the susceptor material comprises a base layer of structural stock material (paperboard); a layer of electrically conductive susceptor material (elemental aluminum); and a protective support material (polyester) for supporting the susceptor material and for contacting the food product. Meanwhile U.S. Pat. Nos. 4,661,672; 4,703,148; and 4,777,053 each disclose packages for heating food in a microwave oven comprising spaced heating and shielding elements which serve both to heat the surface of the food while shielding portions of the food.
However, in accordance with the present invention, the use of a composite susceptor material with multiple susceptor layers in overlying relation, each having their own transmittance and reflectance characteristics, it is possible to control the total amount of energy absorbed for heating, and transmitted for direct cooking, with greater accuracy and more versatility than the prior art. For instance, all susceptors have measurable transmittance and reflectance characteristics. As an example, the absorbance of a single susceptor material having a transmittance of 20% and an reflectance of 20% is 60%. However, the transmittance of a composite susceptor material according to the present invention with two spaced susceptor layers of the same susceptor material is less than 5%, while the absorbed energy of the composite material is increased from 60% to over 75%. Obviously the total energy absorbed or transmitted by the composite susceptor material can be selectively controlled by choosing the susceptor layers having the desired transmittance and reflectance characteristics.